Many disposable absorbent articles use pulp fluff material as the absorbent core. Such cores are generally soft, flexible and absorbent but tend to be bulky and thick and have poor wicking properties. In addition, pulp fluff cores have poor structural stability, prone to collapsing when wet.
An absorbent structure that has poor wicking properties may increase the likelihood of failure of the absorbent product to hold and contain body fluids. Body fluids will be localized to a certain area of a poorly wicking absorbent core, causing saturation in such area whereby excess fluid may overflow through an external surface of the absorbent product. This overflow may contact the user's garment and cause stains or contact the user's body and cause wet discomfort or rash. It is therefore desirable to provide an absorbent core for disposable absorbent articles which can wick away body fluids from the point of contact with the absorbent core and spread it through the absorbent core to more efficiently utilize the entire surface area of the absorbent core. The improved wicking properties of such an absorbent core provide the capacity for fluids to travel by capillary action throughout the surface area of the absorbent core and thus permit the use of thinner cores, since more absorbent surface area can be made available for absorbing body fluids by such wicking action. Thinner absorbent cores may prove to be more comfortable for the user and less unsightly or obvious when worn under clothes.
Absorbent cores with excellent wicking properties comprising peat moss and wood pulp composite materials are described, for example, in U.S. Pat. Nos. 4,170,515; 4,226,237; 4,215,692; 4,507,112; 4,676,871; and 4,473,440. In accordance with the teaching of these patents, an absorbent structure comprising peat moss as a primary absorbent component is formed as a sheet by air or wet laying of fibers and calendering the sheet to obtain a relatively thin, i.e. from about 0.01 to 0.1 inch thick, a relatively dense, i.e. from about 0.2 to 1.0 gram per cubic centimeter (g/cc) structure. Such absorbent peat moss sheets may be processed to increase their flexibility by subjecting the sheets to mechanical tenderizing such as perf-embossing as described in U.S. Pat. No. 4,596,567 or microcorrugating as described in U.S. Pat. No. 4,605,402.
The peat moss sheets thus formed have a large proportion of extremely tiny pores and capillaries which give them the ability to absorb and retain an enormous capacity of fluid. The peat moss pores swell as they absorb fluid, however, this swelling does not cause a loss of capacity for further absorbing fluid. Rather, the swelling contributes to the ability of the sheet to retain fluid while generally maintaining the structural integrity of the absorbent structure in use.
The wicking properties of the above-described peat moss sheets provide the ability for the sheets to be highly absorbent and thin. The flexibility of peat moss sheets may be improved by perf-embossing and/or microcorrugating as described above.
While peat moss sheets make excellent absorbent and wicking cores for disposable absorbent articles, they have limitations. Peat moss sheets may not be readily available particularly in areas which lack the critical raw material, i.e. peat moss or sphagnum moss of desirable age, structure and moisture content. Peat moss sheets also are relatively dark in color and may not be aesthetically acceptable for use in all absorbent products.
Having regard to the foregoing, it is desirable to provide a thin, absorbent and wicking core for disposable absorbent articles which may be substituted for peat moss sheets or pulp fluff sheets.
Attempts to utilize other cellulosic pulp materials such as Kraft wood pulp boards as absorbent cores have not been successful because they tend not to have as much absorbent capacity as peat moss composite sheets but more importantly Kraft wood pulp boards cannot be sufficiently softened for their intended use. While the flexibility and other characteristics of such Kraft wood pulp boards may be improved by perf-embossing and/or microcorrugating techniques, such products still do not provide a desirable combination of absorption capacity and fluid penetration, wicking rates and most importantly a sufficient degree of flexibility for optimal use in disposable absorbent products, particularly sanitary napkins.